A Multi-mode modem can support two or more modem functions. For example, Wireless Wide Area Network (WWAN) radio technologies [e.g., technologies like Global System for Mobile communications (GSM), Code Division Multiple Access (CDMA), Universal Mobile Telecommunications System (UMTS), Long Term Evolution (LTE), Worldwide Interoperability for Microwave Access (WiMAX)], Bluetooth/Wireless Local Area Network (BT/WLAN) radio technology, technology based on Frequency Modulation (FM), and Global Positioning System (GPS) technology can all coexist and operate independently on the same multi-mode modem. In such scenario, a signal transmitted according to a first radio technology (i.e., Mode 1 operation) may cause interference into a signal received according to a second radio technology (i.e., Mode 2 operation) that may be different from the first radio technology. For example, the Wideband CDMA (WCDMA) transmission signal can cause a broadband interference in the GPS receiver operation and desensitize the receiving GPS signal.
The interference may be often generated due to the presence of spur frequency components in outputs of local oscillators (LOs) that perform up and down translations between baseband and radio frequency (RF) signals. This interference may need to be cancelled in order to maintain peak performance in all operation modes.
The typical cause of interference in, for example, multi-mode WAN/GPS modems is the presence of spur frequency components either in the GPS down-converter LO output or in the WAN up-converter LO output. Sources of the spur frequency components can be also: a clock of analog-to-digital converter (ADC), a clock of Temperature Compensated Crystal Oscillator (TCXO) and a clock of frequency modulator (FM) at a transmitter. For example, the ADC clock sourced from RF circuitry can create strong spur frequency components since it requires a higher power clock driver.
One possible solution for canceling the interference in multi-mode modems is to carefully design all hardware components to prevent coupling and improve isolation. However, this is often hard to simulate and test before actual implementation, and sufficient interference cancellation is not guaranteed due to various design uncertainties.
Another possible solution for cancelling the interference in multi-mode modems can be to change the frequency of transmission signal that causes the interference (i.e., changing the aggressor frequency). However, this approach may not always be practical as mobile terminals are often required to transmit in assigned frequency bands.
Yet another solution for cancelling the interference can be to change frequencies of the spur sources so that the interference caused by the spur frequencies fall outside the received signal band. However, there can be unavoidable spur frequencies and interference components generated as the system grows, for example when the standard GPS receiver is extended to operate in a Wideband GPS (WBGPS) mode. In general, if a higher level of system integration is achieved, then more sources of spur frequencies can be present, and therefore it is impractical to change frequencies of a portion of spur sources.